This invention relates to solid state image sensors that include color filter arrays that are formed by a bottom antireflection coating (BARC) color filter process, and methods of making the same.
In general, digital imaging systems, such as digital cameras, include image sensors (or simply imagers) for capturing images. Various types of image sensors have been developed, including charge-coupled device (CCD) image sensors and complementary metal-oxide semiconductor (CMOS) image sensors. These devices typically include an array of pixels, each of which contains a light-sensing element, such as an n+ to p-substrate photodiode, a virtual gate buried n-channel photodetector, or a photo-gate detector, which defines a light-sensing region of an image sensor. Image sensors also include circuitry for driving light signals from the light-sensing elements to other process circuitry. CCD image sensors typically include a photoelectric converter and charge accumulator for absorbing light from an object and collecting photo-generated charges into signal charge packets. In addition, CCD image sensors may include a charge transfer region for conveying charge packets from the photoelectric converter and charge accumulator, and a charge-to-voltage signal converter for generating a voltage output corresponding to the signal charge packets that are transferred through the charge transfer region. CMOS image sensors typically include an array of active pixel sensors and a row (register) of correlated double-sampling (CDS) amplifiers that sample and hold the output of a given row of pixel sensors. In both CMOS and CCD image sensor systems, each pixel sensor accumulates charge during an optical integration period in accordance with the light intensity reaching the relevant sensing area of the pixel sensor.
In color applications, each pixel sensor element typically receives light through a color filter that allows only a relatively narrow radiation wavelength range (e.g., the visible spectrum) to reach the pixel sensors of the image sensor. Multiple sets of color filters typically are arranged in a pattern of pixel size mosaics or pixel wide stripes. The color filters may be applied directly to the surface of an image sensor. Alternatively, the color filters may be formed on a passivation layer (see, e.g., U.S. Pat. No. 5,654,202), in which case a separate masking step is required to expose the bonding pads of the image sensor. The color filters typically are formed from a photoresist structure that includes a layer for each filter color. A common color filter material is spin coated-, dyed-, or pigmented-photoresist. The filter colors for a given color filter set may be additive (e.g., red, green, blue) or subtractive (e.g., cyan, magenta, yellow), or a combination of both additive and subtractive.
The light collecting efficiency of an image sensor may be improved by depositing a micro lens array over the CFA material of each pixel region. A planarization layer that is highly transmissive in the imaging wavelength range also may be deposited between the color filter array and the micro lens material.
The invention features a novel image sensor system and methods of making such a system. In particular, the novel image sensor system includes a color filter array that is formed by an inventive color filter process that incorporates a bottom antireflection coating. The bottom antireflection coating forms a protective layer that protects exposed areas of the active image sensing device structure during formation of the color filter array and, thereby, preserves the intrinsic transmission characteristics of the active image sensing device structure. For example, the bottom antireflection coating protects sensitive areas of the active image sensing device structure against degradation that otherwise might be caused by exposure to developing solutions that are used to pattern the color filter array. The bottom antireflection coating also reduces degradation of metal structures (e.g., bonding pads) and pixel edges at the exposed surface of the active image sensing device structure. For example, the bottom antireflection coating reduces scumming and chemical reactions that otherwise might occur at such metal structures and pixel edges as a result of repeated exposure to color filter resist material and color filter developing solutions. In addition, the bottom antireflection coating provides a uniform adhesive surface for the color filter array, substantially eliminating lifting of the color filter array resist structures. In some embodiments, the bottom antireflection coating also improves the optical transmission characteristics of one or more colors of the colors filter array.
In one aspect of the invention, a bottom antireflection coating is formed over an exposed surface of an active image sensing device structure, a color filter array is formed on the bottom antireflection coating, and exposed portions of the bottom antireflection coating are substantially removed.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The bottom antireflection coating may comprise a dyed organic film-forming material or a light-absorbing polymeric film-forming material.
The bottom antireflection coating preferably has a thickness that is selected to improve optical transmission characteristics of one or more colors of the color filter array. In addition, the bottom antireflection coating preferably is substantially transmissive to radiation in a wavelength range of about 400 nm to about 700 nm.
In some embodiments, the color filter array comprises a plurality of colored photoresist structures.
Exposed portions of the bottom antireflection coating preferably are removed substantially by a plasma etch process (e.g., a low-power buffered oxygen ash process). The plasma etch process preferably removes the bottom antireflection coating at a substantially higher etch rate than the color filter array.
In some embodiments, the bottom antireflection coating forms a substantially continuous layer over the exposed surface of the active image sensing device structure before exposed portions of the bottom antireflection coating are substantially removed. The bottom antireflection coating may form a protective barrier over metal structures at the exposed surface of the active image sensing device structure during formation of the color filter array.
The active image sensor device structure may be a complementary metal-oxide-semiconductor (CMOS) image sensor or a charged-coupled device (CCD) image sensor.
In another aspect, the invention features an image sensor system that includes an active image sensing device structure, a color filter array, and a bottom antireflection coating that is disposed between the color filter array and a surface of the active image sensing device structure.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.